It’s been a while since I played GTA3 so I can’t really remember the best places to camp out and create carnage, but for Vice City I always have luck in a couple places:
The second floor of Sunshine Autos. Shoot out the windows and just start picking off people and cars in the road below. You will eventually get the FBI and Army to show up, and you never have to worry about helicopters. Of course, helicopters are half the fun…
The deck of one of the ships in the port. Just run up the ramp, look out into the dockyard/road area and start wasting people. You do have to worry about choppers here, so make sure you’re packing a rocket launcher. Great place to steal tanks. If you have a sniper rifle, you can usually take out the tank drivers through their front windows.
The low roof right next to the gate entrance to the docks. It’s a little triangular flat roof to the right of the gate if you are heading south down the main road. You can get up there by parking a Packer (those trucks with the ramps on the back) and jumping from the Packer’s ramp to the roof. I like to take a headhsot on a truck driver and stop his truck right below me, then watch the traffic back up behind him. It’s like shooting fish in a barrel. Eventually the helicopters show up and you can spew mayhem to your heart’s content.
I eventually got to the point where I couldn’t fit anymore tanks in any of my garages.
One word of advice though: I strongly recommend getting as many hidden packages as you can. Once you get enough to spawn the rocket launcher at your hideout(s), your chances of surviving the helicopters improve dramatically. Throughout the course of the game you usually wind up at your hideout a bunch of times, so before you know it you have rockets by the dozens.